In the Rails 3 docs, the build method for associations is described as being the same as the new method, but with the automatic assignment of the foreign key. Straight from the docs: Firm#clients.build (similar to Client.new("firm_id" => id)) I've read similar elsewhere. However, when I use new (e.g. some_firm.clients.new without any parameters), the new client's firm_id association is automatical
![Build vs new in Rails 3](https://cdn-ak-scissors.b.st-hatena.com/image/square/98d6f053a97a87156775f60757c60865d0f2c47d/height=288;version=1;width=512/https%3A%2F%2Fcdn.sstatic.net%2FSites%2Fstackoverflow%2FImg%2Fapple-touch-icon%402.png%3Fv%3D73d79a89bded)